Polylactic acid is produced from starch which is a recycled material and is an environmentally friendly resin because of the biodegradability. Products produced using polylactic acid can be disposed of by landfill as compost because of the biodegradability.
Although polystyrene foam molded articles are frequently used as e.g. food containers and cushioning packaging materials, they are needed to be collected for disposal or be recycled by reason of the non-biodegradability.
Patent document 1 describes a method of producing a foam molded article by adding polyalcohols such as glycerin, erythritol and pentaerythritol or polycarboxylic acids such as trimellitic acid and pyromellitic acid to polylactic acid which is substantially amorphous, and cross-linking by polyisocyanate to increase the molecular weight.
In order to obtain a polylactic acid composition having stable melt viscosity, a condition to satisfy the following formula is thought to be good.(0.5×n−100EMi)Mc/10NMi≦W≦(0.5×n−100EMi)Mc/NMi (wherein E: the number of terminal carboxyl groups of polylactic acid (eq/gr)x: the amount of an isocyanate compound to be added (wt %)n: the number of functional groups of the isocyanate compound (eq/gr)Mi: the molecular weight of the isocyanate compound (gr)W: the amount of a polyalcohol or a polycarboxylic acid to be added (wt %)N: the number of functional groups of the polyalcohol or polycarboxylic acid (eq/mol)Mc: the molecular weight of the polyalcohol or polycarboxylic acid (gr))
It is thought that when only polylactic acid is used, a stable composition with high melt viscosity which is appropriate for foam moldings is hard to be obtained.
Also, it is thought to be appropriate when the compounding amount of polyisocyanate is 0.3 to 3% by weight and preferably 0.7 to 2% by weight. When polyisocyanate is insufficient, since a molecular weight of polylactic acid after reactions is too low, only foam molded products with a low expansion ratio can be obtained. On the other hand, it is thought that when polyisocyanate is excessive, good foam molded products cannot be obtained because gelation occurs.
A foam molded article is foam-molded as follows: after impregnated with a foaming agent, pre-expanded beads are transferred and poured into a mold, and the beads are heated with steam to foam. As foaming agents, hydrocarbons such as propane, n-butane, iso-butane, n-pentane, iso-pentane, neopentane, cyclopentane and hexane; halogenated hydrocarbons such as chloromethane, methylene chloride and dichlorodifluoromethane; and ethers such as dimethylether and methylethylether are used, and as auxiliary foaming agents, alcohols having 1 to 4 carbon atoms, ketones, ethers, benzene, toluene and the like are used.
Patent Document 2 describes that a foam nucleating agent is preferably combined to form uniform and fine foam cells, and the foam nucleating agents to be used are preferably particulate solids e.g. inorganic particles such as talc, silica, kaoline, zeolite, mica and alumina; carbonate or bicarbonate; salts such as alkali metal salts of carboxylic acids.
In the production method described in Patent Document 1 in which polyalcohols such as glycerin, erythritol and pentaerythritol or polycarboxylic acids such as trimellitic acid and pyromellitic acid are added, polyisocyanate and polylactic acid are mixed to react in the first step and polyalcohols or polycarboxylic acids are necessarily mixed to react in the second step. It is thought that if the order of the reactions is reversed or the reactions are carried out simultaneously, gelation occurs.
In order that the polyalcohols and polycarboxylic acids are added to react in the second step of the above-described production method, additional equipment investment is required and the production process takes longer, thus this is an economic disadvantage.
Also, there is a serious drawback in that heat-resistance of the foam molded article is insufficient because amorphous polylactic acid is used.